The present invention relates generally to magnetic field absorption, and more particularly, to a ferrite encapsulate coating used for magnetic field absorption.
Metal-based housings are traditionally utilized to protect electronic circuitry from magnetic field interference. Although these devices effectively accomplish the task, the additional cost and added bulk of metal-based housings frequently prevent their incorporation into a final product. Many different products and methods have evolved which improve on both the cost and bulk issues of metal-based housings, but these products and methods also have their own limitations.
One such device is discussed in U.S. Pat. No. 4,474,676, which discloses a shielding material partially consisting of ferrite flakes. The shielding material typically is disbursed into the plastic housing of a device that emits electromagnetic interference (EMI), such as a computer CPU. The shielding material in the plastic housing reduces the amount of EMI radiated into external surroundings. While the material disclosed in ""676 may be effective at reducing EMI emissions, it does not absorb magnetic radiation. Instead it reduces the amount of EMI radiated into the environment.
A second mechanism to reduce electromagnetic interference without cost and space limitations is addressed in U.S. Pat. No. 5,547,599, which discloses an epoxy film. The epoxy film is comprised mainly of a random distribution of ferrite particles in powder form. In its characteristic usage, the epoxy film covers word straps, which are conductive fixtures, on plated wire memory devices and protects them from EMI radiation emitted by external sources. Once the epoxy film covers the word straps, it must be heat cured. Because the epoxy film requires heat curing, it is ill suited for application on heat sensitive components. An additional limitation concerns the random distribution of the ferrite particles after the film is cured. Because the ferrite particles are not concentrated in the locations on the word straps that are most affected by EMI radiation, the epoxy film""s ability to absorb the magnetic radiation is reduced.
U.S. Pat. No. 6,063,511 discloses a typically ferrite-based film to absorb EMI radiation. The film of ""511 is sprayed onto a surface susceptible to EMI radiation. The thickness of most ferrite flakes in this film must be less than the skin depth of all frequencies of EMI radiation that need to be protected against. One limitation of this film is that only the predetermined frequencies of EMI radiation are protected against; thus, any component covered with this film is vulnerable to EMI radiation of an unanticipated frequency. Further, as with ""599, because the ferrite flakes"" location is fixed, they are not able to travel to the location on the component that is most affected by EMI radiation.
Because of the disadvantages of the prior art, it is apparent that a new method for absorbing magnetic field interference is needed. This new method should adequately absorb magnetic field interference on the component needing protection. The new method should also give the absorbing material the ability to migrate to the location on the component most affected by magnetic field interference. Finally, the new method should allow for several means of curing the medium containing the ferrite particles. The present invention is directed to meeting these ends.
One object of the invention is to provide an improved and reliable means to absorb a magnetic field. Another object of the invention is to decrease the costs associated with providing a means to absorb a magnetic field. A third object of the invention is to use ferrite particle migration in order to achieve a more efficient magnetic field absorbing device.
In one aspect of the invention, a method to absorb magnetic field interference begins by placing an electronic unit, such as a seat control module, into a fixture. The electronic unit contains at least one electrical component, such as a microchip, requiring the reduction of a magnetic field. The microchip can be surrounded by a containment apparatus, such as a mold, into which encapsulant is poured. In addition, exclusion devices, such as masks, protect components that should not be coated with encapsulant. Once the electronic unit is prepared, it is exposed to at least one source of magnetic field interference. At this point, encapsulant is poured into all molds on the electronic unit. Ferrite particles comprise a portion of the encapsulant and are randomly distributed throughout. When the ferrite particles are exposed to magnetic field interference, they migrate along the generated field lines and absorb the magnetic field interference. After the new distribution of ferrite particles occurs, the encapsulant can be cured. Curing the encapsulant causes the ferrite particles to be frozen in place along the field lines, allowing for continuous protection of the microchip.
The present invention thus achieves an improved method to absorb a magnetic field. The present invention is advantageous because it is less expensive and occupies considerably less space than metal-based magnetic field absorbing devices. The present invention also minimizes heat buildup on the encapsulate-coated components.
Additional advantages and features of the present invention will become apparent from the description that follows and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.